The present invention relates to a mobile communication system, and particularly to a handover technique for mobile or portable telephone systems (cellular systems) using a direct spread code division multiple access (DS-CDMA) system. More particularly, it relates to a transmit power control of down links for transmission from plural base stations to a mobile station during soft handover where the mobile station is simultaneously connected to the plural base stations.
A conventional technique well known in the art is a cellular system using a code division multiple access (CDMA) system conforming a North American TIA/EIA IS-95 standard. This IS-95 standard uses soft handover. In a system using such a soft handover technique, when a mobile station approaches a boundary between cells (or sectors), the mobile station simultaneusly communicates with plural base stations each having one of the cells as its service area.
When the mobile station detects a base station or stations at reception levels larger than the base station currently in communication therewith, the mobile station regards itself as approaching a cell boundary and starts communication with plural base stations including the currently connected base station and other base stations detected at the larger reception levels. The mobile station then receives the same downward information from the plural base stations, where the downward information from the plural base stations is received at the mobile station by a maximal ratio combining diversity.
Upward information transmitted from the mobile station is received at the plural base stations. These received signals are maximal ratio combined or one of these signal is by a base station controller (BSC) which supervises the plural base stations.
In general, any mobile station located near a cell boundary is likely far at a distance from the base station in the cell, and the reception level of downward signals from the base station would be low. Further, the channel quality is likely deteriorated near the cell boundary due to radio interference from plural cells. Such deterioration of the channel quality can be prevented by connecting the mobile station to plural base stations (by a soft handover).
In a conventional technique such as one conforming the IS-95 standard, the downward signals are simultaneously transmitted from plural base stations during soft handover. Such a system extremely increases the number of currently used down links compared with cellular systems not using the soft handover. An increase in the number of mobile stations during soft handover limits the number of links that can be used simultaneously because of a limited capacity of down links to cause a latent problem that efficiency in channel utilization can not be improved.
Although the IS-95 standard seems to use such an ineffective method that it transmits downward signals from plural base stations, the capacity of down links has not caused any problem because the transmission system of down links has been more effective than that of up links.
However, the capacity of up links is now improving as much as that of down links, and the deterioration in the capacity of down links due to the soft handover needs to be solved.